Murine typhus is a febrile, flea-borne rickettsial disease caused by an obligate intracellular bacterium, Rickettsia typhi. The illness is accompanied by fever, headache, chills, vomiting, nausea, myalgia, and rash and leads to disseminated multisystem disease. Due to its resemblance to diseases like typhoid- or dengue fever it is often misdiagnosed and under reported. Murine typhus is a debilitating disease with over 70% of patients requiring hospitalization. Since the disease does not effectively respond to broad-spectrum antibiotics misdiagnosis and treatment delays increase the risk for complications (i.e., seizures, respiratory and kidney failure, and persistent frontal and temporal lobe dysfunction) or death. Murine typhus is prevalent worldwide and its transmission is closely associated with the human habitations. Importantly murine typhus is endemic in the continental US and in particular is re-emerging in southern Texas and California where the current level of reported human cases is continuing to occur with high prevalence. Murine typhus cases are also on the rise among urban homeless populations, as well as returning travelers from endemic areas. During the previous funding period, we have made several important discoveries concerning the characterizations and functional analysis of R. typhi secreted proteins and host innate immune responses to rickettsial colonization. Our proposed research in this renewal application underscores the role of the R. typhi secretory proteins in the host during the infectio process. The aims of this competitive renewal are now narrowly focused on defining the role of R. typhi secretory proteins that facilitate rickettsial colonization and survival in both flea and at hosts (Aim 1). Based on our published work and preliminary data, we will target R. typhi surface cell antigens, ankyrin repeat proteins and patatin-like phospholipases. Our investigation will cover the infection process in both flea and rat hosts. The two host systems (invertebrate and vertebrate) provide unique insight into dynamics of molecular interactions by which R. typhi target, colonize and survive within the epithelial cells in fleas and endothelial cells in the mammalian host. Specific Aim 2 will explore further how microbe-associated molecular patterns and downstream immune effectors affect Rickettsia typhi colonization in flea hosts. Since differential immune response in infected flea supports the symbiotic stability between this arthropod vector and R. typhi we hypothesize that disrupting the molecular balance between R. typhi and the flea vector should affect flea fitness during infection.